So I guess that the condition (way->oneway) is not precise enough...
But "I might be wrong"!

=> Hell I did not send that answer?!
In the meantime I forked and modified the osm2pgrouting git repo. for it to consider the "oneway" tag values "true", "1" and "yes"...
Maybe some more modifications are to come : the osm wiki states something like 'some ways, like motorways, are automatically considered as oneway edges'.
Still, this statement can be taken into account afterwards with a single sql command...
HF!